FluSurvey reveals hidden burden of influenza-like illness across two UK winters

Every winter, influenza sweeps through communities in waves that hospital statistics and GP records can only partially capture. A new analysis of FluSurvey, a participatory surveillance system run by the UK Health Security Agency, offers one of the clearest pictures yet of what happens to ordinary people when flu-like illness strikes—and how well volunteer-reported symptoms track the official signals used to steer public health responses. The study, published in PLOS Digital Health, examined two consecutive winter seasons, 2023-24 and 2024-25, and found that the vast majority of influenza-like illness never reaches a doctor’s office, yet it disrupts daily life on a remarkable scale.

FluSurvey works by recruiting members of the public who agree to complete weekly symptom surveys throughout the respiratory illness season. Rather than waiting for people to seek care, the system asks them directly whether they have experienced fever, cough, and other symptoms consistent with influenza-like illness, or ILI. This approach, sometimes called participatory or crowd-sourced surveillance, has gained prominence since the COVID-19 pandemic demonstrated how quickly digital reporting tools can complement traditional clinical data streams. The new study set out to characterise the wider impact of ILI among participants and to assess how closely their reports correlate with established influenza surveillance systems, including GP consultation rates, hospital admissions, and laboratory-confirmed PCR test positivity.

The analysis included data from 3,057 participants across the two winter seasons. In 2023-24, 2,540 people took part, roughly 63 percent of them female, with a mean age of 60 years. The following season drew 2,273 participants, 64 percent female, with a mean age of 61. The researchers identified 1,868 ILI episodes over the two winters and examined what happened during each one: whether the person sought healthcare, took medication, missed work or school, used a test, or found their daily activities curtailed.

The headline finding is striking in its consistency: only 14 percent of ILI episodes resulted in contact with healthcare services, most frequently a visit to the GP. In other words, roughly six out of seven episodes of flu-like illness were managed entirely outside the formal healthcare system. This has important implications for how surveillance data are interpreted. Traditional systems that rely on GP consultations see only the tip of the iceberg—the small fraction of illness severe enough, or concerning enough, to prompt a clinical visit. FluSurvey, by contrast, captures the full spectrum of community illness, including the mild and moderate cases that never appear in clinical records.

Yet the fact that most episodes avoided the doctor’s office does not mean they were trivial. A large proportion of episodes—89 percent—involved medication use, suggesting that people were actively treating their symptoms with over-the-counter remedies or prescription drugs. Three-quarters of episodes, 75 percent, were reported to have an impact on daily life, and nearly half, 47 percent, resulted in missed school or work. Absenteeism on this scale carries substantial economic and social costs, from lost productivity to disrupted childcare, that are largely invisible to health systems focused on clinical encounters.

One notable shift between the two seasons concerned testing. The frequency with which participants used tests for respiratory illness declined notably in 2024-25 compared with 2023-24. This pattern likely reflects changing habits as the acute phase of the COVID-19 pandemic recedes further into the past, with fewer people reaching for lateral flow tests or seeking PCR confirmation when they fall ill. The decline complicates efforts to track infection rates through testing data alone and underscores the value of symptom-based surveillance that does not depend on test availability or willingness to test.

The second major aim of the study was to determine whether FluSurvey’s volunteer-reported ILI rates move in step with the official signals that inform public health action. The researchers applied established methodologies, including omitting each participant’s first report—a step designed to reduce bias from people joining the survey precisely because they are ill—and weighting the data to match the age and sex structure of England. They then computed cross-correlations between weekly FluSurvey ILI rates and three national surveillance indicators: GP ILI consultations, influenza hospital admissions, and influenza PCR test positivity, at time lags ranging from two weeks ahead to two weeks behind.

The correlations were strong across the board, and their timing tells a coherent story about the natural history of an influenza season. FluSurvey ILI rates predominantly led GP ILI consultations, with a maximum correlation coefficient of 0.73. This makes intuitive sense: people experience symptoms at home before some of them eventually decide to see a doctor, so community symptom reporting provides an early warning signal of rising clinical activity. FluSurvey rates coincided most closely with influenza hospital admissions, reaching a maximum correlation of 0.88, suggesting that peak community illness aligns tightly with the period of greatest pressure on hospitals. Meanwhile, FluSurvey ILI lagged behind influenza PCR test positivity, with a maximum correlation of 0.88, which fits the expectation that laboratory-confirmed cases among tested patients rise before broader community symptoms peak—or that testing is concentrated early in illness episodes among those who choose to test.

For public health officials, these timing relationships matter. A surveillance signal that leads GP consultations by even a week or two can buy valuable time for interventions such as vaccination campaigns, antiviral distribution, and hospital surge planning. The strong correlation with hospital admissions indicates that FluSurvey is not merely detecting trivial sniffles; the aggregate pattern of community-reported symptoms tracks the severe end of the epidemic curve remarkably well. And the fact that the signal holds up across two distinct winter seasons, with different circulating virus profiles and participant cohorts, lends confidence that the findings are robust rather than a one-off artefact.

The study also highlights the complementary nature of participatory surveillance. It does not replace laboratory testing, which provides virological specificity, or clinical reporting, which captures healthcare burden. Instead, it adds a community-level layer that contextualises the clinical data: how much illness is out there that never reaches a clinic, how it affects work and daily life, and how people manage it on their own. As the authors conclude, the majority of ILI reported to FluSurvey does not involve contact with healthcare services, but it carries wider impacts on daily life, and FluSurvey ILI corresponds well with other national influenza surveillance while providing broader context on community illness. In an era when respiratory virus threats demand rapid, granular situational awareness, systems that enlist the public as sensors may prove an increasingly essential part of the surveillance toolkit.

Subject of Research: Participatory surveillance of influenza-like illness in the UK using FluSurvey over the 2023-24 and 2024-25 winter seasons

Article Title: Monitoring influenza-like symptoms in the UK through participatory surveillance: Insights from FluSurvey over two winter seasons (2023-24 and 2024-25)

Article References: Green, R. E., Mellor, J., Rawlinson, C., Waller, E., Abdul Aziz, N., Watson, C. H., & Dabrera, G. (2026). Monitoring influenza-like symptoms in the UK through participatory surveillance: Insights from FluSurvey over two winter seasons (2023-24 and 2024-25). PLOS Digital Health, 5(10), e0001714. https://doi.org/10.1371/journal.pdig.0001714

Image Credits: AI Generated

DOI: 10.1371/journal.pdig.0001714

Keywords: influenza, FluSurvey, participatory surveillance, influenza-like illness, UK Health Security Agency, public health, epidemiology, respiratory viruses, GP consultations, hospital admissions, PCR test positivity, winter seasons